1. Field of the Invention
This invention is concerned with an improved dip-type efflux viscosity cup. The improved cup is particularly useful for controlling the viscosity of rotogravure printing inks in the pressroom and for checking the calibration of automatic viscosity controls.
2. Prior Art
A large number of viscometers have been proposed and are in use for measuring the viscosity of paints, varnishes, lacquers, gravure and flexographis printing inks, and of other fluid products such as adhesives and petroleum oils. A relatively inexpensive and relatively durable type of viscometer which is well adapted to plant use is the dip-type efflux cup, exemplified by the well known Zahn cup, and the equally well known Shell cup. The Zahn cup is described, for example, on page 1349 of the Sargent-Welch Catalog of Scientific Laboratory Instruments, and on page 1271 of the 1981 Fisher Scientific Catalog. This cup consists essentially of a cylindrical cup with a hemispherical bottom and has a nominal volume of about 44 ml and, concentric with the cylindrical axis, an orifice which varies from about 80 to about 210 thousands of an inch, permitting a range of measurement of about 14 to about 13 hundred centipoises. The cup is provided with a long, looped handle and holding ring and it may be provided also with a thermometer. One of the disadvantages of the cup is that the orifice generally has no protection from accidental mechanical damage, for example, and therefore is subject to change in calibration.
In use, the Zahn cup is chosen having an orifice such that the liquid to be tested has an efflux time of less than about one minute. The cup is immersed into the liquid to be tested, and the viscosity is measured by raising the cup out of the liquid and determining the length of time required for the stream to break.
The Shell cup is similar to the Zahn cup except that it utilizes a capillary tube normally protected by a lip and constructed integrally with the body of the cup. It is altogether somewhat more rugged, but the protecting lip may result in the entrapment of dirt or minute air bubbles and cause distortion of the flow pattern from the capillary and create reproducibility problems.
A modification of the Shell cup, which will be referred to herein as the "S cup", provides a protective collar around the orifice which extends about 2 (two) mm beyond the orifice of the capillary. With the S-cup, the orifice has the same diameter as the capillary itself, i.e., there is no lip around the orifice. Unlike the usual Shell and Zahn cups, the S-cup is free-standing and may be stored on a shelf or table without damage to the exit orifice of the capillary.
The present invention was motivated by repeated complaints from the pressroom of a rotogravure plant concerning the inability to achieve good light end point-out and maintain solid color density.
It is known that one of the most vital characteristics of a gravure ink, with respect to its performance, is the viscosity at which it is printed. Optimum press viscosity insures consistent quality results in the areas of printability, trapping efficiency, level of gloss, adhesion, color density and drying speed--to mention a few of the most important. Experience in the pressroom indicated that good printability on a coated stock red can be achieved by maintaining the viscosity of the ink between 29 and 28 seconds on a Zahn-1 cup.
A preliminary study of the viscosity control problem indicated that the Zahn cup lacked adequate sensitivity for good control. The S-cup, which is a modified Shell cup, appeared best suited in design and in sensitivity to measure the viscosity of rotogravure inks in the region in question. However, it was noted that the S-cup suffered from excessive scatter of data points.
It is an object of this invention to provide an improved efflux viscosity cup which is rugged in design, well adapted to routine production control in a plant environment, and which offers a combination of good sensitivity and good repeatability from test to test.